Orthogonal frequency division multiplexing (“OFDM”) is a multi-carrier transmission technique that uses orthogonal subcarriers to transmit information within an available spectrum. Since the subcarriers are orthogonal to one another, they may be spaced much more closely together within the available spectrum than, for example, the individual channels in a conventional frequency division multiplexing (“FDM”) system.
In an OFDM system, the subcarriers may be modulated with a low-rate data stream before transmission. It is advantageous to transmit a number of low-rate data streams in parallel instead of a single high-rate stream since low symbol rate schemes suffer less from inter-symbol interference (“ISI”) caused by multipath fading. For this reason, many modern digital communications systems are turning to OFDM as a modulation scheme for signals that need to survive in environments having multipath or strong interference. Many transmission standards have already adopted the OFDM system, including the IEEE 802.11a standard, the Digital Video Broadcasting Terrestrial (“DVB-T”), the Digital Audio Broadcast (“DAB”), and the Digital Television Broadcast (“T-DMB”).
At the transmitter side for OFDM signals, the data is encoded, interleaved, and modulated to form data symbols. Overhead information is added, such as pilot symbols. The symbols (data plus overhead) are organized into OFDM symbols. Each symbol is allocated to represent a component of a different orthogonal frequency. An inverse fast Fourier transform (“IFFT”) is applied to the OFDM symbol to generate time samples. Cyclic extensions are then added to the signal and passed through a digital-to-analog converter. Finally, the transmitter transmits the signal to a receiver through a channel (e.g., over the air).
When the receiver receives the signal, the inverse operations are performed on the received signal. The received signal is passed through to an analog-to-digital converter. Also, any cyclic extensions are removed from the signal. The receiver performs a Fast-Fourier transform (“FFT”) on the received signal to recover the frequency components of the signal, i.e., the data symbols. Error correction may be applied to the data symbols to compensate for variations in phase and amplitude caused during the propagation of the signal along the channel. The data symbols are then demodulated, de-interleaved, and decoded, to yield the transmitted data.
The variations in phase and amplitude resulting from propagation along the channel are referred to as the channel response. The channel response is usually frequency and time dependent. If the receiver can determine the channel response, the received signal can be corrected to compensate for the channel degradation. The determination of the channel response is called channel estimation. The inclusion of pilot symbols in each OFDM symbol and/or in each subcarrier allows the receiver to carry out channel estimation. The pilot symbols are transmitted with a value known to the receiver. When the receiver receives the OFDM symbol, the receiver compares the received value of the pilot symbols with the known transmitted value of the pilot symbols to estimate the channel response.
Since the channel response can vary with time and with frequency, the pilot symbols are scattered amongst the data symbols to provide as complete a range as possible of channel response over time and frequency. The set of frequencies and times at which pilot symbols are inserted is referred to as a pilot pattern. FIG. 1 illustrates a pilot distribution pattern of the DVB-T system. Referring to FIG. 1, the pilot symbols are scattered amongst the data carrier symbols along the time domain (y-axis) and the frequency domain (x-axis) forming a grid to enable two-dimensional interpolation. The solid black circles represent the pilot symbols, and the empty circles represent the data carrier symbols. The variable K represents the number of subcarriers in the frequency domain. As an example, for DVB-T systems, in the 2K mode, there are a total of 1705 subcarriers; and in the 8K mode there are a total of 6817 subcarriers.
FIG. 2 illustrates a flow chart of the prior art for channel estimation. For channel estimation 24, the channel transfer function H(m,n) at the pilot cells is computed 40 as a function of the FFT output of the signal. Next, time-domain interpolation 42 is applied, providing time-domain interpolator coefficients. The output of that time domain interpolation is inputted to a frequency domain interpolation 44. The frequency-domain interpolation 44 computes the channel transfer function estimates at all non-pilot tones. After frequency-domain interpolation, the estimates of all the transfer functions at all the data carriers are available either through frequency-domain interpolation for all the non-pilot tones or the time-domain interpolator for the non-pilot cells of all the scattered pilot tones. The channel estimation results can be sent to error correction and/or further processing of the received signal.
This prior art method takes advantage of the scattered pilot position. Interpolation in the time domain is performed first to increase the available carriers for interpolation in frequency domain. As such, this method is suitable for a static environment, where carrier statistics do not change between several symbols in time. However, such method is not suitable for a mobile environment, where the carrier statistics can change significantly, even within a span of a few symbols. Therefore, it is desirable to provide new methods for channel estimation that are suitable for a mobile environment, as well as static environments.